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A210959
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Triangle read by rows in which row n lists the divisors of n starting with 1, n, the second smallest divisor of n, the second largest divisor of n, the third smallest divisor of n, the third largest divisor of n, and so on.
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26
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1, 1, 2, 1, 3, 1, 4, 2, 1, 5, 1, 6, 2, 3, 1, 7, 1, 8, 2, 4, 1, 9, 3, 1, 10, 2, 5, 1, 11, 1, 12, 2, 6, 3, 4, 1, 13, 1, 14, 2, 7, 1, 15, 3, 5, 1, 16, 2, 8, 4, 1, 17, 1, 18, 2, 9, 3, 6, 1, 19, 1, 20, 2, 10, 4, 5, 1, 21, 3, 7, 1, 22, 2, 11, 1, 23, 1, 24
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OFFSET
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1,3
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COMMENTS
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A two-dimensional arrangement of squares has the property that the number of vertices in row n equals the number of divisors of n. So T(n,k) is represented in the structure as the k-th vertex of row n (see the illustration of initial terms).
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LINKS
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EXAMPLE
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Written as an irregular triangle the sequence begins:
1;
1, 2;
1, 3;
1, 4, 2;
1, 5;
1, 6, 2, 3;
1, 7;
1, 8, 2, 4;
1, 9, 3;
1, 10, 2, 5;
1, 11;
1, 12, 2, 6, 3, 4;
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PROG
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(PARI) row(n) = my(d=divisors(n)); vector(#d, k, if (k % 2, d[(k+1)/2], d[#d-k/2+1])); \\ Michel Marcus, Jun 20 2019
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CROSSREFS
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KEYWORD
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nonn,tabf
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AUTHOR
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STATUS
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approved
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